This invention relates to a process for preparing alkyl (meth)acrylates.
Distillation is commonly used in butyl acrylate (BA) production. For example, U.S. Pat. No. 4,012,439 ('439) describes a continuous acrylic acid alkyl ester (“alkyl acrylate”) process in which an esterification mixture produced in a standalone reactor is distilled through an acrylic acid (AA) separation column to give an overhead mixture of alkyl acrylate, butanol, and water, and, from the column bottom, a concentrated AA stream that is returned to the reactor. While separating the overhead mixture from AA, the '439 process recycles a very high proportion (about 97%) of aqueous phase distillate to the head of the AA-separating column. This high proportion of aqueous recycle (i.e. an aqueous reflux ratio of about 32:1) disadvantageously requires a large column and a large expenditure of energy due to the large volumes of water returned to the process.
U.S. Pat. No. 6,180,819 ('819) describes a continuous process for producing alkyl acrylate in which a reactor esterification mixture produced in a standalone reactor is first condensed, then distilled through an AA separation column to give an overhead mixture of alkyl acrylate, butanol, and water, and, from the column bottom, a concentrated AA stream that is returned to the reactor. To separate the overhead mixture from AA and achieve overhead mixture of alkyl acrylate that is substantially free of AA (<2,000 ppm), the '819 process in one embodiment employs a high aqueous reflux ratio of at least 8.5 to the head of the AA-separating column. In fact, '819 demonstrates that using aqueous reflux ratios below 8.5, such as 6.3, leads to an undesirable organic operating state in the AA-separating column characterized by the loss of AA/alkyl acrylate separation where a concentration of AA in the overhead mixture of alkyl acrylate is at least an order of magnitude larger than the maximum of 2,000 ppm AA achieved at reflux ratios of 8.5 and above. Although a substantial improvement over the aqueous reflux ratio of 32 described in '439, the '819 process limit of aqueous reflux ratio of 8.5 disadvantageously requires a large column and a large expenditure of energy associated with returning large volumes of water to the process. Furthermore, the intermediate condensation of the reactor vapor, as well as the use of the standalone reactor and AA-separating column in '819 disadvantageously requires a large capital expenditure for multiple pieces of equipment.
A second embodiment of the '819 process is described wherein esterification mixture vapor is fed directly from the reactor to the AA-separating column, eliminating the need for intermediate condensation of the reactor esterification mixture. However, this embodiment of the '819 process requires an aqueous reflux ratio of at least 13 in order to maintain the desired aqueous operating mode in the AA-separating column to produce overhead mixture of alkyl acrylate that is substantially free of AA (<2,000 ppm). The increased aqueous reflux ratio requirement in this second embodiment of the '819 process, again, disadvantageously requires a large column and a large expenditure of energy due to the large volumes of water returned to the process.
Thus, in the production of alkyl acrylates, there remain problems concerning significant energy use and high capital expenditure. There is a need for a process that would efficiently use the water of reaction in facilitating distillative separation of acrylic ester from AA, particularly with reduced energy use. In addition, there is need for a process for production of acrylates with reduced capital requirements. Meeting one or more of these needs would provide increases in process and/or material use efficiencies.